This invention relates to a back-up clamp for use in the welding of pipelines.
Pipelines are in widespread use for the transportation of fluids such as water, gas and oil. Many such pipelines are of metal and are manufactured in lengths which must be assembled on site into the complete pipeline. To connect two adjacent lengths of pipe together it is usual to provide a welded joint. Thus, a new length of pipe is placed close to the end of the existing pipeline and a welded joint is formed therebetween (it is usual for a small gap to be present between the adjacent pipe ends prior to welding, which gap is filled by the weld). A subsequent length of pipe is then placed against the new end of the (extended) pipeline, and the procedure is repeated.
To ensure an accurate and effective weld it is necessary that the two adjacent pipe ends be maintained in alignment during welding. In addition, a known concern with such welded joints is that weld material can be spattered through the gap and into the interior of the pipe, and can impair the subsequent flow of fluid therealong. It is therefore known to insert a back-up clamp into the pipeline adjacent the joint to be welded, the clamp serving to align the two pipe ends and also having a number of shoes which can be moved to lie beneath the gap and prevent any weld spatter from entering into the pipeline. Copper has been found to be a preferred material for the shoes, and in particular copper xe2x80x9c101xe2x80x9d.
A back-up clamp of this type is disclosed in WO 95/21721. The back-up clamp comprises two sets of clamping members which are adapted to engage the respective pipes adjacent their ends and to assist the alignment of the pipe ends. Between the clamping members is a ring of copper shoes which is adapted to lie underneath the gap during welding. The clamping members and the copper shoes are movable between a retracted condition in which the back-up clamp can be moved along the pipeline, and an extended condition in which the clamping members and the copper shoes engage the inside surface of the respective pipes, the copper shoes lying immediately beneath the gap. G.B. Patent Application 2,067,945, U.S. Pat. No. 2,780,194, U.S. Pat. No. 4,177,914, U.S. Pat. No. 5,110,031 and European patent application 0 193 812 also disclose back-up clamps of the general type described.
The pipelines which require welding may have a diameter ranging from around six inches (15 cm) to around sixty inches (150 cm), with the back-up clamps having diameters corresponding to slightly less than the diameter of the pipeline. The devices described in the above patents and applications are suited for use with larger diameter pipelines.
The means for actuating the known back-up clamps, i.e. for moving the clamping members and the back-up shoes, is typically by way of hydraulic or pneumatic fluid, which fluid is delivered to the device along the pipeline by a series of flexible hoses. The clamp itself will typically carry a large number of flexible hoses, each end of the hoses being connected to respective parts of the clamp, so that the hose can communicate pneumatic or hydraulic fluid from one location on the clamp to another location on the clamp. For example, if as is typical the back-up clamp has a set of clamping members to either side of the back-up shoes it is necessary to transmit pressurised fluid past the back-up shoes to the actuating means for the set of clamping members remote from the end of the pipeline. Since the back-up shoes engage the pipeline around its full circumference it is not possible to pass the fluid around the back-up shoes, and the fluid must instead pass between those shoes.
It is known to provide a rigid tube upon the clamp which tube passes between adjacent back-up shoes and carries one or more conduits for pressurised fluid; at each end of the tube the conduits terminate in connectors, and flexible hoses are connected to the respective connectors to communicate the fluid to the actuating means for the clamping members. Clearly, in more complex clamps, a greater number conduits may be required within the tube, and a greater number of flexible hoses required for connection thereto. In one known design, the tube can contain six conduits, for example.
Typically therefore, back-up clamps of the types described in the prior art documents listed above require a number of flexible hoses to either side of the back-up shoes. With clamps for larger-diameter pipelines there is usually enough space to accommodate the hoses and their connections to the clamp, but this becomes increasingly difficult to achieve as the diameter of the pipeline, and thus the diameter of the clamp, becomes smaller. Thus, notwithstanding that the hoses can be reduced in cross-sectional size as the clamp becomes smaller, the requirement to connect the hoses to chosen locations on the clamp, and for the hoses to pass around and between other components of the clamp, provides a physical limitation on the smallest diameter which can be provided for, and this smallest diameter is around eight inches (20 cm).
Such clamps are therefore not suited for use on the many piplines having diameters smaller than around eight inches. In addition, the provision of flexible hoses has a number of specific disadvantages.
Firstly, great care has to be taken, both during use of the clamp and also during on-site and off-site handling, to ensure that no damage is caused to any of the hoses. If any damage is caused to a hose, this might not become apparent until the clamp is in use, perhaps preventing one set of the clamping members from operating for example; this will result in considerable wastage of time whilst the clamp is retrieved from within the pipeline, and repaired.
Secondly, the repair of a damaged hose requires a replacement hose to be available, and this might not always be the case on site.
Thirdly, the requirement that a hose may need to be replaced on site requires the hose connections to be releasable, and typically threaded connections are used. Cheaper, permanent, connections cannot be used. In addition, the hose connections must all be made in positions which are accessible to a spanner or other tool, which positions might not be ideal for clamp operation. The result is that the clamp is more complex, and expensive, to manufacture than would be the case if permanent connections could be provided.
Fourthly, it is necessary that the hoses be correctly connected so that the fluid is communicated to the correct location of the clamp. Typically the hoses will be connected in the factory where the clamp is manufactured, but in the event that a user has to replace two or more hoses at the same time, great care must be taken to ensure the correct connections.
It is an object of the present invention to provide a back-up clamp which avoids or reduces the disadvantages with the prior art clamps.
According to the invention therefore, there is provided a back-up clamp comprising at least one set of clamping members and a set of back-up shoes, the set of clamping members and back-up shoes being operable by fluid pressure, characterised in that the conduits for transmission of fluid pressure are all located within the body of the clamp.
With the present invention therefore, the clamp itself requires no flexible hoses. The advantages of the invention will be apparent, namely that the specific disadvantages of flexible hoses can be avoided. In addition, however, the physical limitation on the smallest diameter of the clamp is removed, so that a back-up clamp according to the invention can be used on smaller pipelines, for example six inch (15 cm) diameter pipelines, so increasing the utility of back-up clamps for pipelines on which they might otherwise not be suitable. However, notwithstanding the advantages of the invention in providing a smaller-diameter clamp than was previously possible, the invention can be utilised for the largest diameter pipelines, where the avoidance of flexible hoses is a considerable advantage.
Preferably, one end of the back-up clamp has a number of connectors, each connector being adapted to cooperate with a connector on the end of a fluid pressure hose. Thus, notwithstanding that the clamp itself requires no flexible hoses, it is still likely that the hydraulic or pneumatic fluid will be delivered to the clamp by way of such hoses. The advantage of this feature, however, is that all of the hoses can be connected to one end of the clamp, and connection of the hoses is a relatively quick and simple task, unencumbered by surrounding clamp componentry. None of the fitted hoses is required to pass around or between any of the clamp componentry.
Preferably, the back-up clamp has two sets of clamping members, one located to either side of the back-up shoes.
Preferably, the back-up clamp includes a pair of flexible bladder means adapted to engage the inside of the pipeline and to provide an air-tight seal therewith. With such embodiments, the volume around the weld-site can be filled with an inert gas such as argon, which is required for the effective welding of certain materials such as stainless steel.
Desirably, the back-up clamp includes a through-bore between its opposed ends. The through-bore permits the equalisation of pressure between the opposed ends of the clamp, and so reduces the likelihood of forced movement of the back-up clamp which might otherwise occur in the presence of a large pressure differential to either side of the clamp. Large pressure differentials are known to be caused for example when pipelines are laid by ship, movement of the ship relative to the laid pipeline compressing (or stretching) the pipeline to one side of the clamp and changing the pressure of the air therewithin. Significant changes in pressure to one side only of the clamp can force the clamp along the pipeline, out of its required position.
Preferably, the back-up clamp is made up in sections, the sections having conduit(s) formed therethrough to permit the passage of fluid. Each section can have a number of conduits which are substantially aligned with the longitudinal axis of the clamp (and so serve to communicate fluid pressure to the next section).
Desirably, some of the sections can have a number of passageways which can communicate fluid pressure from one radial position to a different radial position (so that the conduit can communicate fluid pressure to an actuating cylinder for example).
Accordingly, it is preferred that the pressurised fluid pass along conduits within the clamp at a chosen radius, and only deviate from that radius at the position along the clamp at which fluid within that conduit is required to undertake work such as drive a piston.
Desirably, the clamping members and the back-up shoes are independently actuated. Desirably, each is actuated by its own piston and cylinder arrangement. Usefully, each piston and cylinder arrangement actuates the respective clamping members or back-up shoes by way of a respective toggle mechanism.
Desirably, the piston of each piston and cylinder arrangement can move longitudinally relative to the clamp, the toggle mechanism converting the longitudinal movement of the piston into radial movement of the clamping members or back-up shoes, as the case may be.
Alternatively, at least the back-up shoes are actuated by radially-acting pistons, the clamp having means to communicate fluid pressure to drive the pistons radially outwardly. Desirably, separate means are provided to communicate fluid pressure to drive the pistons radially inwardly also.